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PHYSICS OF THE AIR

in the cyclone. Frequently, as explained above, the cirro-stratus is only the higher and swifter portion of the cyclonic cloud system, the result of forced convection to great altitudes.

Cirro-cumulus (Ci.-Cu.).—Cirro-cumuli are small, fleecy cumulus clouds, generally 6 to 7 kilometers above the surface; that is, in the lower cirrus region. They usually occur in large numbers, producing an effect sometimes described as "curdled sky;" frequently, also, in groups and rows that remind one of the patterns (not the scales) on the backs of

FIG. 83.—Cirro-cumuli. (F. Ellcrman, photographer.)

mackerel. Hence the expression "mackerel-back sky/' commonly abbreviated to "mackerel sky" (Fig. 83).

Their origin obviously is due, chiefly, to a single cause—local vertical convections, induced perhaps by an overrunning cold, or underflowing warm, layer of air, or by the lift or drop of a thin cloud sheet, itself changing temperature according to the wet adiabatic rate and the clear air according to the dry. When an unstable fluid sheet is thin, it breaks up into numerous convectional regions, the theory of which, for liquids, has been developed by Lord Rayleigh;1 a theory that, if extended to include gases, might also, as Brunt2 has suggested, account for the formation of all layers of closely set, but essentially isolated, clouds,

lPhil Mag., 32; 529, 1916.

2 Meteorological Mag., 60; 1, 1925.